First, concerning the mold for thermoplastic injection molding and the cavity mold relating to the present invention, examples of the conventional techniques will be described below.
FIG. 8 is a schematic sectional view of a mold for injection-molding including a concave cavity mold 102 and a convex core mold 108. FIG. 9 is an enlarged view of the part IX of FIG. 8. In this cavity mold, a thin metallic layer 103 is formed on the surface of a heat-insulating layer 101 made of heat-resistant plastic.
This cavity mold 102 is manufactured as follows.
First, a mother die which has the same outside dimensions as those of the cavity part (cavity) 105, that is, the molding shape is formed, and on its surface, a thin metallic layer 103 is formed by electroforming, injected and fixed into a cavity mold member via heat-resistant plastic, and molding a heat-insulating layer 101 at the same time, and then the mother die is removed, whereby the cavity mold 102 is manufactured.
On the surface of a core mold 108, a mold release heat-insulating layer 104 made of fluorine resin-dispersed nickel plating is formed.
The heat insulating layer 101 and the thin metallic layer 103 are joined to the cavity mold by an adhesive, etc., to form the surface of the cavity mold 102.
A molten resin from an injection molding machine passes through a spool 106 and a gate 107 from a nozzle touch portion 110 and is then filled in the cavity part (cavity) 105 formed between the cavity mold 102 and the core mold 108.
In the figures, the reference numeral 109 denotes an attaching plate, 111 denotes a projecting pin, 112 denotes a cavity plate, and 113 denotes a core plate.
Normally, a mold for injection molding is made of a metallic material, and when a resin is filled in the mold and comes into contact with the mold surface, this filling progresses while a solidified layer is formed on the resin surface, and after completion of the filling, a predetermined pressure is applied, however, not only does the formed solidified layer hinder transfer of the mold surface, but also a turbulence and an irregular flow are frozen on the solidified layer and causes a weld and a flow mark in many cases.
Herein, “weld” means a stripe pattern occurring at a junction of molten resin flows inside the mold, and “flow mark” means a pattern like a ripple formed on the surface of a molding due to resin flows.
To solve the above-described problem, an injection molding heat-insulating mold (abbreviated to heat-insulating mold or heat-insulating die) is devised. The heat-insulating cavity mold is structured so that on the surface of a normal cavity mold, a heat-insulating layer is formed, and on the surface thereof, a thin metallic layer is further formed so as to make the heat capacity of the mold surface small.
When a resin is filled in the heat-insulating mold, the cavity surface is instantaneously raised in temperature due to the quantity of heat of the resin, and thereafter, the heat is quickly released and the temperature is lowered, so that the appearance is improved without losing productivity, and the problem of a normal mold is reduced or solved.
Some methods for producing the heat-insulating cavity mold have been proposed, and according to one method, a metallic layer of 0.1 to 0.5 mm is formed by electroforming nickel on a mother die having the same shape as a molding shape by using an electroforming method, and on the surface thereof, a heat-insulating layer is provided and a backing block is further backed and integrated, and then the mother die is removed, whereby a heat-insulating cavity mold is produced (Patent document 4).
There is another method in which a heat-insulating layer is formed by resin coating, etc., on the surface of a cavity mold made of a normal steel material, and thereon, electroless plating is further formed (Patent document 1).
According to the former electroforming method, a thin metallic surface layer is formed on a mother die first, and a heat-insulating layer is provided thereon, and the latter method is a pile-up method in which a heat-insulating layer is formed on the mold surface first, and a metallic layer is provided thereon, and the steps are reversed.
A heat-insulating mold is proposed which includes a heat-insulating cavity mold formed by the above-described electroforming and a core mold coated with a mold release heat-insulating coating (Patent document 3), and hereinafter, a heat-insulating mold formed by a conventional method or electroforming means an injection molding heat-insulating mold based on this Patent document 3.    Patent document 1: Japanese Published Unexamined Patent Application No. H07-137040    Patent document 2: Japanese Published Unexamined Patent Application No. 2002-172655    Patent document 3: Japanese Published Unexamined Patent Application No. H06-218769    Patent document 4: Japanese Published Unexamined Patent Application No. S55-55818    Non-patent document 1: “Development of accurate transfer and sink preventive low-pressure molding system by melt movement control when cooling-melt moving molding method,” Japan Society of Polymer Processing 2000, Lecture meeting collected papers A207 from 11th Workshop, pp. 47    Non-patent document 2: “Measurement of heat-insulating mold surface-melt adhesion influencing melt movement,” Meeting of Japan Society of Polymer Processing 2004, Lecture meeting collected papers VI-205 from 15th annual meeting, pp. 309